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Open SoC Debug

Overview

Stefan Wallentowitz - November 12, 2015

What is Open SoC Debug

  • Hardware building blocks for SoC Debugging

  • Infrastructure for debug systems for system-on-chip around those building blocks

  • Basic host libraries to interact with the debug system

Why Open SoC Debug

  • Benefit from existing building blocks

  • Interoperability with other SoCs

  • Efficient debug systems become more important

System Overview

Modular design, clear interfaces, composability

Physical Transport

  • Not defined: Physical interface and protocols

  • Use Generic Logic Interface Project (glip) as transport

  • One common interface for different interfaces

  • Wrappers around open source interfaces

  • Currently: USB 2/3 (with Cypress FX), JTAG, PCIE

  • Simple FIFO interface, usually 16-bit

Transport Interface (glip)

Hardware:

wire [WIDTH-1:0] data;
wire valid;
wire ready;

Software:

void glip_send(size_t len, uint16_t* data);

// poll
int glip_receive(size_t maxlen, uint16_t* data);

// callback
glip_register_callback(void (*recv)(size_t maxlen, uint16_t* data));

Debug Packets

  • Control, data and events are transmitted as packets

  • Each module and the host have a unique ID

  • Basic Format:

Debug Network-on-Chip

  • Packets are routed over a NoC

  • Baseline is a simple, uni-directional ring

  • Low cost

  • Arrange positions with traffic demands

  • Each module and host are participants

Debug Modules

  • Common interface: NoC packets

  • Modules can consume and produce packets

  • Convenience wrapper: Memory-mapped registers

  • Common basic functions: version, capabilities, enumeration

  • Share a timebase via a globally distributed counter

Core Debug Modules

  • Implement traditional run-control debugging

  • Control functions: Run, step, reset

  • Data functions: Read and write registers

  • Breakpoint functions: Set and enable

  • Generate breakpoint event

Core Trace Modules

  • Emit debug events of execution trace

  • Configurable

  • On each instruction, branch, etc.

  • Compression mechanism

  • Set trigger conditions to reduce trace sizes

Trace Cross-Triggering

  • Correlate trigger events

  • Ease debugging of complex systems

  • "Trace this core if the DMA takes to long"

  • "Trace the interconnect load during synchronization events"

  • Separate trigger network for precise timing correlation

Debug Processors

  • Debug modules with an entire subsystem

  • Configure trace events to go there

  • Aggregate and filter events

  • Small microporgrams to genrate information from data

  • Can generate trace events or even control messages

Trace Recording

  • Standard: online debugging

  • Possible multiplexed trace targets, e.g.,

  • Trace to SD card

  • Trace to system memory

System Control Module

  • Common module to all open soc debug systems

  • Platform/Chip identifier

  • Possibly clock gating for emulation mode

  • Overflow configuration:

  • Gate clocks (care needed with respect to I/O)

  • Drop packets and notify host about this

Other Debug Modules

  • Trace and debug devices

  • Memory access module (read, write memory)

  • Interconnect load traces

  • ...

Host Interface

  • Lowest level: Send a packet to a debug module
void osd_send(uint16_t module, uint8_t class,
             size_t len, uint16_t *packet) {
    int16_t dbgpacket[MAX_DBGPKT_LEN];
    assert((len+2) <= MAX_DBGPKT_LEN);

    dbg_packet[0] = module << 6;
    dbg_packet[1] = (OSD_HOST_ID << 6) | (class & 0x1f);
    memcpy(&dbg_packet[2], packet, len);

    glip_send(len + 2, dbg_packet);
}

Host Interface: Memory-mapped

void osd_mm_read(uint16_t module, uint16_t address,
                 uint16_t *value) {
   int rv;
   int16_t packet[1];

   packet[0] = address;
   osd_send(module, OSD_CLASS_MMRD, 2, packet);

   osd_receive(module, OSD_CLASS_MMRD, 1, packet);
   *value = packet[0];
}

Host Debug Software

  • opensocdebug library as interface to debug infrastructure

  • Low-level and memory-mapped functions

  • APIs for different debug modules

  • Integrate with (open) debug software

Get involved

Looking for people to discuss, write spec and contribute.

[email protected]